1. Field of the Invention
The present invention relates to an image read-out apparatus for reading out image information by illuminating an original.
2. Description of the Prior Art
Conventionally, photo reaction type sensors made of amorphous silicon material have been used. The sensor made of such material exhibits a change in resistance responsive to the intensity of the light emitted onto the sensor. Therefore, by constituting a circuit so that this resistance change is detected using an electrical circuit, the intensity of the light to be emitted onto the sensor can be output as an electrical signal.
However, this type of sensor has the problem that, when the light is emitted thereon, its sensitive characteristic gradually deteriorates in proportion to the time it is subjected to the light emission. Consequently, there is the problem that if the light is continuously emitted onto all sensors while the signals of the sensors are being read out, it will cause deterioration in the sensitivity of the sensors and will result in a loss of their function as sensors for a short time.
To solve this problem, conventionally, a method whereby the time period whent he light is emitted to the sensor is minimized as needed thereby to prevent deterioration sensitivity of the sensor, has been generally used.
A constitution of a conventional sensor and a light source section will be described hereinbelow with reference to drawings.
FIG. 1 is a cross sectional view showing a constitution of a sensor and a light source section of an image read-out apparatus, in which a reference numeral 1 denotes a sensor made of amorphous silicon material. Light condensing material 2 serves to focus the light from a light source, 3 which may as shown comprise light source units on opposite sides of light condensing material 2 reflected from a read-out point P on an original 4 into the sensor 1.
FIG. 2 diagrammatically illustrates an arrangement of the sensor and light source in FIG. 1, in which numerals 1 and 3 are common to those in FIG. 1. The sensor 1 is constituted by many blocks (SB1, SB2, . . .) each consisting of four sensor elements (S1-S4, S5-S8 . . .). The light source 3 is also constituted of many blocks represented by D1 through Dn (where n is a natural number). In this light source 3, the size of each block Di (where i is a natural number from 1 to n) is set to be substanitally equal to a size of one block of the sensor.
Conventioally, in case of fetching the outputs of one block of the sensor, e.g., of the sensor elements S1 to S4, only the light source of block D2 was lit. With such a constitution, the time when the sensor is irradiated can be set into a factor of total number of blocks. Namely, the quantity of light to be emitted ot the sensor 1 can be minimized, thereby enabling deterioration in sensitivity of the sensor to be retarded by a corresponding amount.
However, in this method, since the light source block and sensor block have the same size, the quantity of light ot be emitted to the sensor is lower at both ends of each sensor block than in the middle, as shown in a curve A in FIG. 3, so that there are drawbacks such as that this causes the sensor output to be distorted and an appropriate signal cannot be obtained and, accordingly, it is necessary to correct the signal by providing a distortion correction circuit or the like, resulting in the complication fo the signal fetching circuit.
On the other hand, there is known an image read-out sensor which is constituted by an array of optical sensors such as CCDs (charge coupled devices) arranged fully in the location corresponding to a whole width of an original in the facsimile apparatus or an apparatus for entering characters, picture, etc. Such a read-out sensor together with a light source arranged similarly along the width of original constitutes an image read-out apparatus and serves to read out a picture on the original by means of the light read out a picture on the original by means of the light from the light source to be reflected through the original. In recent years, there has been frequently used an arrangement wherein a number of LEDs used as the light source are arranged in a line near the read-out sensor for the purpsoes of miniaturization and decrease in weight of the apparatus.
In the case of such a light source using LEDs, in many cases, the ligth source is arranged so as to exceed the original width to compensate for a decrease in the quantity of light at both end portions of the original. For examle, in the case of an original size of A4 (210 mm in width), the light source is arranged soa s to extend an additional 10 mm or so, at both ends beyond the arrangement width of the read-out sensor which is equal to the width of the original. Thus, the whole length of the light source made up of the LEDs is about 230 mm. In this case, if the arrangement pitch of the LEDs is set to be so small as about 1.25 mm to prevent deterioration in light quantity distribution characteristic due to variation in the quantity output by light of each LED, a total of 184 LEDs will be needed. When it is now assumed that electric power consumption per LED is 40 mW, then when all LEDs are lit, heat generation of total 7 W or more would be produced.
In general, as for LEDs, denoting the luminous intensity at an ambient temperature of 25.degree. C. by 1, the luminous intensity at 50.degree. C. is about 20%. Furthermore, a quantity of light to be emitted from the LED also deteriorates even due to the heat generation of the LED itself. Therefore, radiation fins are ordinarily attached to the light source using LEDs of this kind to suppress the influence of the heat. As a result, the size of a light source section inevitably becomes large, causes a problem regarding miniaturization and decrease in the weight of the apparatus.
In consideration of the above drawbacks, light source control as shown in FIGS. 4A-4E is often performed in the conventional apparatus.
The photo sensor 1 and light source 3 are controlled with timings as shown in FIGS. 4A-4E. As shown in FIG. 4A, the blocks SB1 to SB4 of the photo sensor 1 are sequentially enabled one by one and each light source block D2 to D5 is also sequentially lit and controlled synchronously with this. (However, although each sensor block SB1-SB4 is strobed for every block, each photo sensor element in a block is sequentially enabled.)
While the amount of heat generation in the light source can be advantageously suppressed by performing such light-on control, there are drawbacks such as tht the quantity of light is lower at both end portions of each block, which occurs at both end portions of the light source in the case of simultaneously lighting all of the light sources, and that an error will be easily caused when the signal read out by the photo sensor 1 is subjected to binary-code processing at the post stage.